1. Technical Field
The present invention relates to an electromagnetic flow meter, and more particularly, to a two-frequency-excitation-type electromagnetic flow meter that detects the non-full level of a fluid to be measured.
2. Description of the Related Art
In flow control performed in chemical plants, as the excitation type of an electromagnetic flow meter that is used to measure the flow rate of a fluid, generally, a composite excitation type (hereinafter, a ‘two-frequency excitation type’) has been known in which an excitation current component with a high frequency (first frequency) and an excitation current component with a frequency (second frequency) lower than the first frequency flow through an exciting coil at the same time to form a composite magnetic field. FIG. 8 is a diagram illustrating the structure of a two-frequency-excitation-type electromagnetic flow meter 1. The structure and operation of the electromagnetic flow meter 1 will be described with reference to FIG. 1.
In FIG. 8, the electromagnetic flow meter 1 includes a detector 10, an exciting circuit 20, an amplifying circuit 30, an A/D (analog/digital) converter 31, a constant current circuit 40, and a CPU (central processing unit) 50.
The detector 10 includes an exciting coil 11 and electrodes 12 and 13. The CPU 50 includes a high frequency flow rate calculating unit 51, a low frequency flow rate calculating unit 52, a two-frequency flow rate calculating unit 53, a non-full level detecting unit 54, and an output unit 55.
The electrodes 12 and 13 are provided in the detector 10, and the exciting coil 11 is provided such that the magnetic field generated from the electrodes is applied to a fluid R to be measured in the detector 10.
The outputs of the electrodes 12 and 13 are input to the amplifying circuit 30, and the amplifying circuit 30 amplifies the difference between the outputs of the electrodes 12 and 13 and outputs the amplified signal to the A/D converter 31. The A/D converter 31 converts the differential amplification signal into a digital signal and outputs the digital signal to the CPU 50.
An output terminal of the constant current circuit 40 is connected to the electrodes 12 and 13. For example, the constant current circuit 40 includes two diodes (not shown). An anode of each diode is connected to a predetermined voltage and a cathode thereof is connected to the electrode 12 or 13. In the constant current circuit 40, a leakage current (hereinafter, referred to as a ‘constant current’) flows to the electrodes 12 and 13 in the opposite direction of the diode.
The high frequency flow rate calculating unit 51 and the low frequency flow rate calculating unit 52 in the CPU 50 receive the digital signal from the A/D converter 31 and calculate the flow rate of the fluid R to be measured corresponding to the excitation frequency.
The two-frequency flow rate calculating unit 53 receives the flow rates calculated by the high frequency flow rate calculating unit 51 and the low frequency flow rate calculating unit 52 and calculates the flow rate of the fluid R to be measured corresponding to two-frequency excitation.
The non-full level detecting unit 54 receives the outputs of the electrodes 12 and 13 when a constant current flows from the constant current circuit 40 to the electrodes 12 and 13 through the amplifying circuit 30 and the A/D converter 31 and detects whether the fluid R to be measured is at a non-full level in the detector 10.
The output unit 55 receives the flow rate calculated by the two-frequency flow rate calculating unit 53 and the detection signal detected by the non-full level detecting unit 54. Then, the output unit 55 outputs a current signal that corresponds to the flow rate or indicates the non-full level.
Next, the operation of the electromagnetic flow meter 1 measuring the flow rate and detecting the non-full level will be described. The exciting circuit 20 makes an excitation current (two-frequency excitation current), which is the sum of a high frequency excitation current and a low frequency excitation current, flow to the exciting coil 11 on the basis of the excitation control signal from the CPU 50, thereby generating a magnetic field from the exciting coil 11. The exciting coil 11 applies a magnetic field corresponding to the excitation current to the fluid R to be measured.
The electrodes 12 and 13 detect and output a signal (electromotive force) that corresponds to a flow velocity and the magnetic field and is generated by the magnetic field corresponding to the high frequency excitation current and the low frequency excitation current.
The CPU 50 receives the signals output from the electrodes 12 and 13 through the amplifying circuit 30 and the A/D converter 31.
The high frequency flow rate calculating unit 51 in the CPU 50 performs a predetermined operation on the received signal in synchronization with a high frequency to calculate a flow rate eH (a first flow rate; hereinafter, referred to as a ‘high frequency flow rate’) corresponding to high-frequency excitation. The high frequency flow rate calculating unit 51 performs a low-pass operation on the high frequency flow rate eH to calculate a high frequency low-pass filtered flow rate FH (first low-pass filtered flow rate).
The low frequency flow rate calculating unit 52 performs a predetermined operation on the received signal in synchronization with a low frequency to calculate a flow rate eL (a second flow rate; hereinafter, referred to as a ‘low frequency flow rate’) corresponding to low-frequency excitation. The low frequency flow rate calculating unit 52 performs a low-pass operation on the low frequency flow rate eL to calculate a low frequency low-pass filtered flow rate FL (second low-pass filtered flow rate).
The two-frequency flow rate calculating unit 53 adds the high frequency low-pass filtered flow rate FH and the low frequency low-pass filtered flow rate FL in synchronization with the high frequency to calculate a flow rate eA (a third flow rate; hereinafter, referred to as a ‘two-frequency flow rate’) corresponding to two-frequency excitation.
The output unit 55 outputs a current signal (for example, in the range of 4 to 20 mA) or a voltage signal (for example, in the range of 1 to 5 V) corresponding to the two-frequency flow rate eA.
The following two methods are used to detect whether the fluid is at a non-full level in the non-full level detecting unit 54.
(1) First, a method of making a constant current flow from the constant current circuit 40 to the electrodes 12 and 13 will be described. When a constant current flows with the fluid R to be measured at a non-full level, the difference (differential voltage) between the output voltages of the electrodes 12 and 13 is higher than that when the fluid is at a full level.
The non-full level detecting unit 54 compares the differential voltage with a predetermined detection voltage. When the differential voltage is higher than the predetermined detection voltage, it is detected that the fluid is at the non-full level. The method of detecting the non-full level is disclosed in JP-A-3-186716.
In addition, an AC coupling circuit (for example, a capacitor (not shown)) for attenuating a DC component may be connected to the outputs of the electrodes 12 and 13. The connection of the capacitor is disclosed in JP-A-6-174513.
(2) A method of detecting whether the fluid is at a non-full level on the basis of a noise component overlapped with the output signals from the electrodes 12 and 13 will be described. In this method, the constant current circuit 40 may not be used.
When the fluid R to be measured is at the non-full level, the level of noise overlapped with the output signals from the electrodes 12 and 13 is more than that when the fluid is at a full level. For example, the noise includes commercial power supply frequency noise and inductive noise generated by the magnetic field generated from the exciting coil 11.
The non-full level detecting unit 54 measures the level (voltage) of noise overlapped with the output signals from the electrodes 12 and 13. When the level of noise is more than a predetermined detection voltage, it is determined that the fluid is at the non-full level. The method of detecting the non-full level is disclosed in JP-A-3-257327 and JP-A-3-60027U.
Next, the operation of the output unit 55 when a non-full level detection signal is received from the non-full level detecting unit 54 will be described.
When the fluid is at the non-full level, the electromagnetic flow meter 1 is in an abnormal state in which it is difficult to accurately measure the flow rate. In this case, in order to notify the abnormal state in which the fluid is at the non-full level to the outside, the output unit 55 outputs a current or voltage signal that is beyond the normal range (hereinafter, referred to as ‘burnout’) or outputs a warning signal, such as warning light or warning sound.
However, the above-mentioned two methods of detecting the non-full level have the following problems.
(1) In the method of making a constant current flow, when the AC coupling circuit is used, the following problems arise. When the fluid is changed from the full level to the non-full level or from the non-full level to the full level, the outputs of the electrodes 12 and 13 vary greatly.
Therefore, it takes a long time for the output of the AC coupling circuit to be stabilized by a differential operation and the non-full level detecting unit 54 detects the full level and the non-full level after the outputs are stabilized. Therefore, it takes a long time to detect the full level and the non-full level (for example, about 10 minutes).
(2) In the method of detecting the full level and the non-full level from the noise component, for example, when the level of noise generated by a noise source is reduced by a surrounding environment, the level of noise overlapped with the output signals from the electrodes 12 and 13 is less than a predetermined detection voltage even when the fluid is at the non-full level. Therefore, it is difficult for the non-full level detecting unit 54 to detect the non-full level.
In this case, the output unit 55 does not receive a non-full level detection signal. Therefore, the output unit 55 outputs a current or voltage signal corresponding to the two-frequency flow rate eA without burning out the current or the voltage. However, actually, since the fluid is at the non-full level and the output signals from the electrodes 12 and 13 vary greatly, a hunting phenomenon in which the current or voltage output alternates between the upper limit and the lower limit occurs.